(a) Field of the Invention
The present invention relates to a mask assembly for a cathode ray tube (CRT), and more particularly, to a mask assembly for a CRT for reducing the amount of electron beam impinging on a shadow mask and thereby preventing doming phenomenon of the shadow mask.
(b) Description of the Related Art
Generally, a color CRT displays images by exciting phosphors with electron beams, and includes a screen on which red, green and blue color emitting phosphors are coated, an electron gun for forming three electron beams, a deflection yoke for directing the electron beams to the screen, and a shadow mask to permit each electron beam to strike only the phosphor elements associated with the beam.
The shadow mask is a thin sheet of metal, and includes a large central apertured portion through which the electron beams pass, a solid border portion surrounding the apertured portion, and a peripheral skirt portion angled from the border portions of the mask and directed to the electron gun, that is directed to opposite direction of a panel of the CRT. The apertures of the shadow mask are formed so as to correspond to the red, green and blue phosphors.
In the conventional shadow mask, approximately 20% of electron beam passes through the apertures formed on the shadow mask, and the remaining 80% of electron beam strikes the shadow mask to generate heat and raise the temperature of the shadow mask. Since the skirt portion of the shadow mask is firmly fixed to a frame of the CRT, the shadow mask is domed when it is heat-expanded. When the shadow mask is domed, the positions of apertures on the shadow mask are moved, and each electron beam can not strike the phosphor elements associated with the beam, but strikes the shadow mask or the phosphor of undesired colors. This results in the deterioration of the color purity of the CRT. The deterioration of the color purity due to the doming of the shadow mask and mis-landing of electron on the screen is generally called as "doming phenomenon".
To prevent or reduce the "doming phenomenon", a shadow mask made of Invar (an iron-nickel alloy) or a shadow mask on which powders of Invar are coated are developed. Since the Invar has very low heat expansion coefficient, the doming phenomenon is more or less reduced by using the Invar as the material of the shadow mask.
However, even though a material of low heat expansion coefficient is used to form shadow mask, the doming phenomenon is not fully reduced. Furthermore, there are disadvantages that the Invar is expensive and separate processing steps and various chemical compounds are required to coat the powder of the Invar on the shadow mask.